两个有序的链表合并为一个有序链表

class Node(object):
    def __init__(self, item):
        self.elem = item
        self.next = None

class SingleLinkList(object):
    def __init__(self,node=None):
        self._head = node

    def travel(self):
        ptr = self._head
        while(ptr!=None):
            print(ptr.elem)
            ptr = ptr.next

    def move(self, step):
        ptr = self._head
        for i in range(step):
            ptr = ptr.next
        return ptr.elem

    def append(self, item):
        node = Node(item)
        if self._head == None:
            self._head = node
        else:
            ptr = self._head
            while (ptr.next != None):
                ptr = ptr.next
            ptr.next = node

    def length(self):
        if self._head == None:
            return 0
        else:
            ptr = self._head
            i = 0
            while (ptr != None):
                ptr = ptr.next
                i += 1
        return i

def merge(sl1, sl2, res):
    len1 = sl1.length()
    len2 = sl2.length()
    i1 = 0
    i2 = 0
    while(1):
        if sl1.move(i1)<=sl2.move(i2):
            res.append(sl1.move(i1))
            i1 += 1
        else:
            res.append(sl2.move(i2))
            i2 += 1
        if i1==len1 and i2!=len2:
            while(1):
                res.append(sl2.move(i2))
                i2 += 1
                if i2==len2-1:
                    return
        elif i1!=len1 and i2==len2:
            while(1):
                res.append(sl1.move(i1))
                i1 += 1
                if i1==len1-1:
                    return
        elif i1 == len1 and i2 == len2:
            return

if __name__=="__main__":
    sl1 = SingleLinkList()
    sl1.append(1)
    sl1.append(3)
    sl1.append(5)
    sl1.append(7)
    sl1.append(9)
    sl1.append(10)
    sl1.append(11)
    sl1.append(12)
    sl1.append(13)

    sl2 = SingleLinkList()
    sl2.append(2)
    sl2.append(4)
    sl2.append(6)
    sl2.append(8)

    res = SingleLinkList()
    merge(sl1, sl2, res)
    res.travel()